The invention refers to a thermal infrared detector of the resistance bolometer type, whereby radiation incident on the detector is absorbed, causing a rise in the temperature of the detector and a change in electrical resistance. This resistance change is observable as a variation in the electrical bias current or voltage applied to the detector.
It must be understood that thin film resistance bolometer infrared detectors have been previously described. Reference may be made to a paper by K. C. Liddiard entitled "Thin Film Resistance Bolometer IR Detectors" published in Infrared Physics, Vol. 24, No. 1, p. 57, January 1984, and other references cited therein. Patents on bolometer detectors are also well known, for example the patents to K. C. Liddiard, Australia No. 537314; U.S. Pat. No. 4,574,263; Canada No. 1184642; Europe No. 0060854. Those patents also cite a number of references on the art.
However, the papers and patents cited refer to metal film bolometer detectors, wherein the heat sensitive material is a thin metal film. These detectors have a tow temperature coefficient of resistance (TCR) and low electrical resistance, which together give very small signal levels in the nanovolt range. Consequently, the infrared responsivity measured as the ratio of signal voltage to incident radiant power is also small, typically less than 100 volts per watt. It is the objective of the present invention to improve the detecting ability by employing a semiconductor film as the heat sensitive material. Both the TCR and electrical resistance are much larger, resulting in signal levels in the microvolt range, with responsivities exceeding 10000 volts per watt. Such high signal levels, together with a smaller power dissipation, make the semiconductor bolometer more suitable for large focal plane arrays.
U.S. Pat. No. 4,116,063 describes a bolometer designed specifically to operate at a very low temperature, and has a sensitive element of a semiconductor crystal extended on two faces by beams of the same material, but of smaller cross-section which have been metallised.
U.S. Pat. No. 3,069,644 is directed to a bolometer comprising an evacuated envelope having a glass frame, a thin film of insulating material with spaced strips of metallic film on the insulating film, and a thin elongated layer of semiconducting material extending across the strip.
A semiconductor film bolometer infrared detector has been described in a paper by K. C. Liddiard titled "Thin Film Resistance Bolometer IR Detectors-II", published in Infrared Physics, Vol. 26, No. 1, p. 43, January 1986. This paper, and other reference cited above do not describe either the method of preparation or the materials technology which are features of the present invention. In particular, the method of preparation whereby a thin film bolometer infrared detector array is prepared by on-the-plane, single-sided monolithic microcircuit processing techniques, has not been previously described.